WO2025079655A1 - Steel sheet - Google Patents

Abstract

Provided is a steel sheet that can achieve both excellent hardenability and excellent cold workability even when a C content exceeds 0.90%. A steel sheet according to the present disclosure contains, in mass %: greater than 0.90-1.30% of C; 0.01-0.50% of Si; 0.20-1.30% of Mn; 0.100% or less of P; 0.100% or less of S; 0.100% or less of Al; 0.01-0.50% of Cr; 0.0150% or less of N; and the remnant being Fe and impurities. The total area ratio of ferrite and cementite particles is 95% or greater. The average particle diameter of the ferrite is 15.0 μm or less. The Cr concentration [Cr]_θ in the cementite particles is 2.65% or less. The Mo concentration [Mo]_θ is 1.30% or less. The average particle diameter of the cementite particles is 1.50 μm or less. The maximum particle diameter of the cementite particles is 5.00 μm or less. The spheroidizing rate of the cementite particles is 75% or greater.

Description

Steel Plate

This disclosure relates to steel sheets.

Steel plate with a high C content (high carbon steel plate) may be used as a material for machine parts such as automobile parts and industrial machinery parts. When manufacturing these machine parts using steel plate as a material, the manufacturing method is as follows. The steel plate is cold worked to process it into an intermediate product in the shape of the machine part. The intermediate product is quenched and tempered. Through the above manufacturing process, high-strength machine parts are manufactured. In order to obtain high strength in the machine parts after quenching, the steel plate is required to have excellent hardenability when quenched during the manufacturing process of the machine parts. Furthermore, the steel plate is cold worked before quenching. Therefore, the steel plate is required to have not only excellent hardenability but also excellent cold workability.

Patent Documents 1 and 2 propose steel sheets that have excellent hardenability during hardening and excellent cold workability when used as materials for machine parts.

The steel sheet disclosed in Patent Document 1 contains, in mass%, C: 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al: 0.10% or less, N: 0.0050% or less, and B: 0.0005 to 0.0050%, and further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in a total amount of 0.002 to 0.030%, with the balance being Fe and unavoidable impurities. In this steel sheet, the proportion of the amount of solid-solubilized B in the B content is 70% or more. Furthermore, the microstructure is composed of ferrite and cementite. Furthermore, the cementite density in the ferrite grains is 0.08 pieces/μm ² or less. Patent Document 1 describes that the total elongation can be increased by keeping the cementite density of ferrite grains low.

The steel plate disclosed in Patent Document 2 contains, by mass%, C: 0.10% to 0.33%, Si: 0.01% to 0.50%, Mn: 0.40% to 1.25%, P: 0.03% to 0.01%, sol. Al: 0.10% to 0.01%, N: 0.01%, and Cr: 0.50% to 1.50%, with the remainder being Fe and unavoidable impurities, and has a microstructure containing ferrite and carbides. The volume ratio of ferrite and carbides to the entire microstructure is 90% or more, and the volume ratio of pro-eutectoid ferrite to the entire microstructure is 20% to 80%. The Mn concentration in the carbide is 0.10% by mass or more and 0.50% by mass or less, and the ratio of the number of carbides with a particle size of 1 μm or more to the total number of carbides is 30% to 60%. Patent Document 2 describes that by reducing the Mn concentration in the carbide, the carbide becomes more easily dissolved during hardening, and as a result, the hardenability is improved.

International Publication No. 2015/146173 International Publication No. 2020/175665

Incidentally, among machine parts, bearings for automobile parts and knitting needles for textile machine parts are required to have high strength and excellent wear resistance. Therefore, the steel plate used as the material is required to have further improved hardenability. As a result, steel plate with a C content of more than 0.90% is sometimes used as steel plate for high-strength machine parts. Even in such steel plate with a high C content, excellent cold workability is required.

The objective of this disclosure is to provide a steel sheet that can achieve both excellent hardenability and excellent cold workability even when the C content exceeds 0.90%.

The steel sheet according to the present disclosure has a chemical composition, in mass%, of C: more than 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.50%, N: 0.0150% or less, Mo: 0 to 0.400%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the balance being Fe and impurities. In the microstructure of the steel sheet, the total area ratio of ferrite and cementite particles is 95% or more, and the average grain size of ferrite is 15.0 μm or less. The Cr concentration [Cr] _θ in mass% in the cementite particles is 2.65% or less, and the Mo concentration [Mo] _θ in mass% in the cementite particles is 1.30% or less. The average particle size of the cementite particles is 1.50 μm or less, and the maximum particle size of the cementite particles is 5.00 μm or less. Among the cementite particles, cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more.

The steel sheet according to the present disclosure has excellent hardenability and excellent cold workability.

FIG. 1 is a schematic diagram for explaining the three-point bending test. FIG. 2 is a graph of the load-stroke curve obtained in the three-point bending test shown in FIG.

The inventors conducted research into steel sheets that have excellent hardenability and excellent cold workability. As a result, the inventors obtained the following findings.

First, the inventors investigated the improvement of hardenability and cold workability in steel plates with a C content of more than 0.90% from the perspective of chemical composition. As a result, the inventors believed that a chemical composition consisting of, in mass%, C: over 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.50%, N: 0.0150% or less, Mo: 0 to 0.400%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance being Fe and impurities, would be able to improve both hardenability and cold workability. Therefore, the inventors further examined the steel plate having the above-mentioned chemical composition from the viewpoint of microstructure.

The inventors first investigated means for improving the hardenability of the microstructure of steel sheet. The microstructure of steel sheet having the above-mentioned chemical composition is substantially composed of ferrite and cementite particles. When manufacturing machine parts using steel sheet as a material, in order to improve the hardenability of the steel sheet during hardening in the manufacturing process of the machine parts, it is preferable that the cementite particles in the steel sheet are easily dissolved during hardening. In order to improve the solid solubility of the cementite particles during hardening, it is preferable that the particle diameter of the cementite particles is small. In the case of steel sheet having the above-mentioned chemical composition, the hardenability is improved if the average particle diameter of the cementite particles is 1.50 μm or less.

Furthermore, as a result of the inventors' investigations, it was found that the Mn concentration in cementite particles does not affect the solid solution of cementite particles during quenching. On the other hand, the inventors discovered that the Cr and Mo concentrations in cementite particles have a significant effect on the solid solution of cementite particles during quenching. Specifically, if the Cr and Mo concentrations in cementite particles are high, the cementite particles are less likely to dissolve during quenching.

Based on the above findings, further studies were carried out, and as a result, the present inventors found that when the Cr concentration [Cr] _θ in the cementite particles is 2.65% or less and the Mo concentration [Mo] _θ is 1.30% or less in mass %, the cementite particles are easily dissolved during heat treatment, and the hardenability of the steel sheet is improved.

The present inventors further investigated means for improving the cold workability of the microstructure of the steel sheet. In order to improve the cold workability of the steel sheet, it is effective to set the average particle size of the cementite particles to 1.50 μm or less and to increase the spheroidization rate of the cementite particles. Furthermore, the average particle size of the ferrite also affects the cold workability. Therefore, in the steel sheet of this embodiment, the average particle size of the cementite particles is set to 1.50 μm or less, the spheroidization rate of the cementite particles is set to 75% or more, and the average particle size of the ferrite is set to 15.0 μm or less. If the average particle size of the ferrite is 15.0 μm or less, the grain boundary area of the ferrite is large. In this case, the dissolution of the cementite particles is accelerated during quenching due to grain boundary diffusion. Therefore, if the average particle size of the ferrite is 15.0 μm or less, not only the cold workability but also the quenchability is improved.

However, even when the steel sheet has the above-mentioned chemical composition, the average particle size of the cementite particles is 1.50 μm or less, the Cr concentration [Cr] _θ in mass % in the cementite particles is 2.65% or less, the Mo concentration [Mo] _θ is 1.30% or less, the spheroidization rate is 75% or more, and the average particle size of ferrite is 15.0 μm or less, there are still cases where sufficient cold workability cannot be obtained. Therefore, the present inventors conducted further studies. As a result, the present inventors obtained the following findings.

If the C content exceeds 0.90%, the size of the cementite particles that form during the cooling process of hot working is likely to vary. Even if the average particle size of the cementite particles is 1.50 μm or less, if the size of the cementite particles varies and coarse cementite is present, the coarse cementite will become the starting point of cracks during cold working. As a result, the cold workability of the steel sheet is reduced.

Based on the above findings, the inventors conducted further investigations. As a result, the inventors discovered that in the above-mentioned steel plate, if the maximum particle size of the cementite particles is 5.00 μm or less, the deterioration of cold workability caused by coarse cementite can be suppressed, and it is possible to achieve both excellent hardenability and excellent cold workability.

The steel plate according to this embodiment, which was completed based on the above findings, has the following configuration.

The steel plate of the first configuration has a chemical composition, in mass%, of C: more than 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.50%, N: 0.0150% or less, Mo: 0 to 0.400%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the balance being Fe and impurities. In the microstructure of the steel plate, the total area ratio of ferrite and cementite particles is 95% or more, and the average grain size of ferrite is 15.0 μm or less. The Cr concentration [Cr] _θ in mass% in the cementite particles is 2.65% or less, and the Mo concentration [Mo] _θ in mass% in the cementite particles is 1.30% or less. The average particle size of the cementite particles is 1.50 μm or less, and the maximum particle size of the cementite particles is 5.00 μm or less. Among the cementite particles, cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more.

The steel plate of the second configuration is a steel plate of the first configuration, and the chemical composition contains, in mass%, one or more elements selected from the group consisting of Mo: 0.001-0.400%, Ni: 0.001-1.000%, B: 0.0001-0.0100%, V: 0.001-0.500%, Nb: 0.001-0.500%, and Ti: 0.001-0.150%.

The steel plate of this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[Features of the steel sheet according to the present embodiment]
The steel sheet of this embodiment satisfies the following features 1 to 7.
(Feature 1)
The chemical composition, in mass%, is C: over 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 0.50%, N: 0.0150% or less, Mo: 0 to 0.400%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance being Fe and impurities.
(Feature 2)
In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more.
(Feature 3)
The average grain size of ferrite is 15.0 μm or less.
(Feature 4)
The Cr concentration [Cr] _θ in terms of mass % in the cementite particles is 2.65% or less, and the Mo concentration [Mo] _θ in terms of mass % in the cementite particles is 1.30% or less.
(Feature 5)
The average particle size of the cementite particles is 1.50 μm or less.
(Feature 6)
The maximum particle size of the cementite particles is 5.00 μm or less.
(Feature 7)
When cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more.
Features 1 to 7 will be explained below.

[Feature 1: Chemical composition]
The chemical composition of the steel sheet of this embodiment contains the following elements.

C: more than 0.90 to 1.30%
Carbon (C) enhances the hardenability of steel plate. As a result, by performing hardening in a process for manufacturing a mechanical part using steel plate as a raw material, the strength of the mechanical part is enhanced. Furthermore, C enhances the wear resistance of the mechanical part by leaving undissolved cementite particles after hardening and tempering in a process for manufacturing a mechanical part using steel plate as a raw material. If the C content is 0.90% or less, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the C content exceeds 1.30%, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is deteriorated, and further, the toughness of machine parts manufactured using the steel sheet as a material is deteriorated.
Therefore, the C content is more than 0.90% to 1.30%.
The lower limit of the C content is preferably 0.91%, more preferably 0.93%, and further preferably 0.95%.
The upper limit of the C content is preferably 1.28%, more preferably 1.25%, and further preferably 1.20%.

Si: 0.01~0.50%
Silicon (Si) deoxidizes steel during the steelmaking stage of the steel sheet manufacturing process. Furthermore, Si enhances the temper softening resistance of the steel sheet when tempering is performed in the process of manufacturing machine parts using the steel sheet as a raw material. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Si content exceeds 0.50%, the strength of the steel sheet becomes excessively high due to solid solution strengthening, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Si content is 0.01 to 0.50%.
The lower limit of the Si content is preferably 0.02%, more preferably 0.05%, and further preferably 0.10%.
The upper limit of the Si content is preferably 0.48%, more preferably 0.44%, and further preferably 0.40%.

Mn: 0.20-1.30%
Manganese (Mn) enhances the hardenability of steel sheets depending on the amount of solid solution in austenite during heating in the hardening process in the process of manufacturing mechanical parts using steel sheets as a raw material. As a result, the strength of mechanical products is increased. If the Mn content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mn content exceeds 1.30%, the strength of the steel sheet becomes excessively high due to solid solution strengthening, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mn content is 0.20 to 1.30%.
The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, and further preferably 0.35%.
The upper limit of the Mn content is preferably 1.25%, more preferably 1.20%, and further preferably 1.15%.

P: 0.100% or less Phosphorus (P) is an impurity, and the P content is more than 0%. If the P content exceeds 0.100%, the toughness of the steel plate decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the P content is 0.100% or less.
The P content is preferably as low as possible. However, excessive reduction in the P content significantly increases the production cost. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
The upper limit of the P content is preferably 0.090%, more preferably 0.080%, and further preferably 0.050%.

S: 0.100% or less Sulfur (S) is an impurity, and the S content is more than 0%. If the S content exceeds 0.100%, S generates excessive sulfides. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced.
Therefore, the S content is 0.100% or less.
The S content is preferably as low as possible. However, excessive reduction in the S content significantly increases the production cost. Therefore, in consideration of industrial production, the lower limit of the S content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
The upper limit of the S content is preferably 0.090%, more preferably 0.080%, still more preferably 0.050%, still more preferably 0.030%, and still more preferably 0.025%.

Al: 0.100% or less Aluminum (Al) is an impurity, and the Al content is more than 0%. Al combines with N to form AlN. AlN refines austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material. Refining austenite grains reduces the hardenability of the steel sheet. If the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains are excessively refined during heating in the quenching process, and the hardenability of the steel sheet is significantly reduced.
Therefore, the Al content is not more than 0.100%.
The lower limit of the Al content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, further preferably 0.070%, and further preferably 0.050%.
In the chemical composition of the steel sheet of this embodiment, the Al content means the acid-soluble Al (sol. Al) content.

Cr:0.01~0.50%
Chromium (Cr) enhances the hardenability of the steel sheet depending on the amount of chromium dissolved in austenite during heating in the hardening process in the process of manufacturing mechanical parts using the steel sheet as a raw material. As a result, the strength of the mechanical parts is increased. If the Cr content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Cr content exceeds 0.50%, it becomes difficult to make the Cr concentration [Cr] _θ in the cementite particles 2.65% or less. In this case, the dissolution of the cementite particles is delayed during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material. As a result, the quenchability is reduced. Furthermore, the strength of the steel sheet becomes excessively high. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced.
Therefore, the Cr content is 0.01 to 0.50%.
The lower limit of the Cr content is preferably 0.02%, more preferably 0.03%, and further preferably 0.05%.
The upper limit of the Cr content is preferably 0.48%, more preferably 0.45%, and further preferably 0.42%.

N: 0.0150% or less Nitrogen (N) is an impurity, and the N content is more than 0%. N combines with Al to form AlN. AlN refines austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material. Refining austenite grains reduces the hardenability of the steel sheet. If the N content exceeds 0.0150%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains are excessively refined during heating in quenching, and the hardenability of the steel sheet is significantly reduced.
Therefore, the N content is 0.0150% or less.
The lower limit of the N content is preferably 0.0001%, and more preferably 0.0005%.
The upper limit of the N content is preferably 0.0140%, more preferably 0.0130%, and further preferably 0.0125%.

The balance of the chemical composition of the steel plate of this embodiment is composed of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the steel plate, and are acceptable to the extent that they do not adversely affect the steel plate of this embodiment.

The impurities may include the following elements in the amounts listed below.
One or more elements selected from the group consisting of Cu: 0-0.15%, W: 0-0.15%, Ta: 0-0.15%, Sn: 0-0.050%, Sb: 0-0.050%, Co: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, and Ca: 0-0.050%. All of these elements are tramp elements, and are impurities in the steel plate of this embodiment.

[Optional Elements]
The chemical composition of the steel sheet of this embodiment may further contain, instead of a portion of Fe, one or more elements selected from the group consisting of Mo: 0-0.400%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, and Ti: 0-0.150%. All of these elements are optional elements. These optional elements will be described below.

[Group 1 (Mo, Ni and B)]
The chemical composition of the steel sheet of the present embodiment may further contain, instead of a portion of Fe, one or more elements selected from the group consisting of Mo, Ni, and B. All of these elements are optional elements and may not be contained. When contained, Mo, Ni, and B improve the hardenability of the steel sheet.

Mo: 0-0.400%
Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0%.
When Mo is contained, that is, when the Mo content is more than 0%, Mo dissolved in austenite during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material increases the hardenability of the steel sheet. Therefore, the strength of the mechanical product increases. Mo also increases the temper softening resistance of the steel sheet. The above effects can be obtained to a certain extent even if even a small amount of Mo is contained.
However, if the Mo content exceeds 0.400%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel sheet becomes excessively high, and therefore the cold workability of the steel sheet deteriorates.
Therefore, the Mo content is 0 to 0.400%.
The lower limit of the Mo content is preferably more than 0%, more preferably 0.001%, further preferably 0.003%, further preferably 0.005%, and further preferably 0.010%.
The upper limit of the Mo content is preferably 0.380%, more preferably 0.370%, further preferably 0.350%, further preferably 0.330%, and further preferably 0.300%.

Ni: 0-1.000%
Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%.
When Ni is contained, that is, when the Ni content is more than 0%, Ni dissolved in austenite during heating in the quenching process in the process of manufacturing mechanical parts using the steel plate as a raw material increases the quenchability of the steel plate. Therefore, the strength of the mechanical parts is increased. Ni also increases the temper softening resistance of the steel plate. The above effects can be obtained to a certain extent even if even a small amount of Ni is contained.
However, if the Ni content exceeds 1.000%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel sheet becomes excessively high, and therefore the cold workability of the steel sheet deteriorates.
Therefore, the Ni content is 0 to 1.000%.
The lower limit of the Ni content is preferably more than 0%, more preferably 0.001%, further preferably 0.005%, further preferably 0.007%, and further preferably 0.010%.
The upper limit of the Ni content is preferably 0.950%, more preferably 0.900%, more preferably 0.870%, more preferably 0.800%, more preferably 0.700%, and more preferably 0.600%.

B: 0-0.0100%
Boron (B) is an optional element and may not be contained, that is, the B content may be 0%.
When B is contained, that is, when the B content is more than 0%, B dissolved in austenite during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material increases the quenchability of the steel sheet, thereby increasing the strength of the mechanical parts. Even if even a small amount of B is contained, the above effect can be obtained to a certain extent.
However, if the B content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, B compounds are generated. In this case, sufficient hardenability cannot be obtained. Furthermore, the cold workability of the steel sheet is deteriorated.
Therefore, the B content is 0 to 0.0100%.
The lower limit of the B content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0003%, and further preferably 0.0005%.
The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, still more preferably 0.0070%, still more preferably 0.0060%, and still more preferably 0.0050%.

[Group 2: V, Nb and Ti]
The chemical composition of the steel plate according to the present embodiment may further contain one or more elements selected from the group consisting of V, Nb, and Ti instead of a portion of Fe. All of these elements are optional elements and may not be contained. When contained, V, Nb, and Ti form carbides and suppress coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using the steel plate as a material. Therefore, the toughness of the mechanical parts is improved.

V: 0-0.500%
Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%.
When V is contained, that is, when the V content is more than 0%, V forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using steel plates as raw materials. Therefore, the toughness of the mechanical parts is improved. Even if even a small amount of V is contained, the above effect can be obtained to a certain extent.
However, if the V content exceeds 0.500%, excessive carbides are formed, which causes precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet is deteriorated even if the contents of other elements are within the ranges of this embodiment.
Therefore, the V content is 0 to 0.500%.
The lower limit of the V content is preferably more than 0%, more preferably 0.001%, further preferably 0.003%, and further preferably 0.005%.
The upper limit of the V content is preferably 0.480%, more preferably 0.450%, further preferably 0.440%, and further preferably 0.400%.

Nb: 0-0.500%
Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%.
When Nb is contained, that is, when the Nb content is more than 0%, Nb forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process of manufacturing mechanical parts using steel plate as a raw material. Therefore, the toughness of the mechanical parts is improved. In addition, Nb bonds with N and suppresses the formation of nitrides by the solute B. This improves the hardenability of the steel plate due to the solute B. Even if even a small amount of Nb is contained, the above effects can be obtained to a certain extent.
However, if the Nb content exceeds 0.500%, the steel sheet is precipitation-strengthened by excessively forming carbides, and therefore the cold workability of the steel sheet is deteriorated even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Nb content is 0 to 0.500%.
The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, further preferably 0.003%, and further preferably 0.005%.
The upper limit of the Nb content is preferably 0.480%, more preferably 0.460%, more preferably 0.430%, more preferably 0.400%, more preferably 0.350%, and more preferably 0.300%.

Ti: 0-0.150%
Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%.
When Ti is contained, that is, when the Ti content is more than 0%, Ti forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process of manufacturing mechanical parts using steel plate as a raw material. Therefore, the toughness of the mechanical parts is improved. Ti also combines with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel plate due to solute B. Even if even a small amount of Ti is contained, the above effects can be obtained to a certain extent.
However, if the Ti content exceeds 0.150%, excessive carbides are formed, which causes precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet is deteriorated even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Ti content is 0 to 0.150%.
The lower limit of the Ti content is preferably more than 0%, more preferably 0.001%, further preferably 0.003%, further preferably 0.005%, and further preferably 0.010%.
The upper limit of the Ti content is preferably 0.148%, more preferably 0.145%, more preferably 0.130%, more preferably 0.120%, more preferably 0.100%, and more preferably 0.080%.

[Method of measuring chemical composition of steel sheet]
The chemical composition of the steel plate of this embodiment can be measured by a known composition analysis method. Specifically, chips are collected from the inside of the steel plate at a depth of 0.1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas fusion-thermal conductivity method.

[Feature 2: Microstructure]
In the microstructure of the steel sheet of the present embodiment, the total area ratio of ferrite and cementite particles is 95% or more, that is, the microstructure of the steel sheet of the present embodiment is substantially composed of ferrite and cementite particles.

In the microstructure, the remaining structure other than the ferrite and cementite particles is, for example, one or more types selected from the group consisting of bainite, martensite, and pearlite.

Preferably, the total area ratio of ferrite and cementite particles in the microstructure is 96% or more, more preferably 97% or more, even more preferably 98% or more, and even more preferably 99% or more. The microstructure may be a structure consisting of ferrite and cementite particles.

If the total area ratio of ferrite and cementite particles is 95% or more, the steel plate will have sufficient hardenability and sufficient cold workability, provided that characteristics 1 and characteristics 3 to 7 are met.

[Method for measuring the total area ratio of ferrite and cementite particles in a microstructure]
The total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method.
A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate. The surface of the test piece that is parallel to the rolling direction and thickness direction (i.e., the surface of 15 mm in the rolling direction and thickness direction) is used as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etching solution). Of the etched observation surface, secondary electron images are observed using a 1000x scanning electron microscope (SEM: Scanning Electron Microscope) at five arbitrary observation fields located at a depth of 4/thickness from the steel plate surface in the thickness direction. Each observation field is a rectangle measuring 100 μm in the thickness direction and 120 μm in the rolling direction. The observation field is selected so that the center position of the observation field in the thickness direction is located at a depth of 4/thickness from the steel plate surface in the thickness direction of the observation surface.

In the observation field, ferrite and cementite particles show different contrast and morphology than other structures (bainite, martensite, pearlite, etc.). Therefore, ferrite and cementite particles are identified in the observation field based on the contrast and morphology.

The total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ferrite and cementite particles in the five observation fields and the total area of the five observation fields (100 μm x 120 μm x 5).

[(Feature 3) Average grain size of ferrite]
Furthermore, in the steel sheet of this embodiment, the average grain size of ferrite is 15.0 μm or less.
If the average grain size of ferrite exceeds 15.0 μm, the grain boundary area is reduced, and the effect of promoting the dissolution of cementite particles by grain boundary diffusion cannot be obtained. In this case, sufficient hardenability cannot be obtained in the hardening process when manufacturing mechanical parts using the steel plate as a raw material. Therefore, the strength of the mechanical parts manufactured using the steel plate as a raw material is reduced.

When the average grain size of ferrite is 15.0 μm or less, the grain boundary area of ferrite is sufficiently large, and the cementite particles dissolve quickly during quenching. Therefore, assuming that characteristics 1, 2, and 4 to 7 are satisfied, the hardenability is improved. Furthermore, when the average grain size of ferrite is 15.0 μm or less, the cold workability of the steel plate is improved.

The upper limit of the average grain size of ferrite is preferably 14.5 μm, more preferably 14.0 μm, even more preferably 13.5 μm, and even more preferably 13.0 μm.
The lower limit of the average grain size of ferrite is preferably 3.0 μm, more preferably 3.5 μm, and even more preferably 4.0 μm.

[Method for measuring average grain size of ferrite]
The average grain size of ferrite can be measured by the following method.
A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate. The surface of the test piece is a cross section parallel to the rolling direction and thickness direction (i.e., the surface of 15 mm in the rolling direction and thickness direction). The observation surface of the test piece is mirror-polished. After mirror-polishing, etching is performed with a 3% nital etching solution. The average grain size of ferrite is determined on the etched observation surface by the following method. In accordance with JIS G 0551:2020, the grain size number of ferrite is determined by a cutting method. At this time, the magnification of the optical microscope is selected so that the number of ferrite grains cut by one line segment is at least 10 or more in one visual field. After selecting the magnification, the cutting length is determined for five visual fields. The grain size number of ferrite is determined from the arithmetic average value of the cutting lengths of the five visual fields. The average grain size (μm) of ferrite is determined from the obtained grain size number. The average grain size of ferrite is calculated by rounding off the obtained value to one decimal place.

[(Feature 4) Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles]
In the steel sheet of this embodiment, the Cr concentration [Cr] _θ in mass % in the cementite grains is 2.65% or less, and the Mo concentration [Mo] _θ in mass % in the cementite grains is 1.30% or less.

When the Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles of the steel sheet are high, the cementite particles do not dissolve sufficiently during heating in the quenching process of manufacturing a mechanical part using the steel sheet as a material. In this case, the quenchability of the steel sheet is reduced. As a result, the mechanical part manufactured using the steel sheet as a material does not have sufficient strength.

When the Cr concentration [Cr] _θ in the cementite particles of the steel sheet is 2.65% or less and the Mo concentration [Mo] _θ in the cementite particles is 1.30% or less, the Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles are sufficiently low. Therefore, on the premise that Features 1 to 3 and Features 5 to 7 are satisfied, the cementite particles are sufficiently dissolved during heating in the above-mentioned quenching step, and the hardenability of the steel sheet is improved.

The upper limit of the Cr concentration [Cr] _θ in the cementite grains is preferably 2.60%, more preferably 2.50%, and still more preferably 2.40%.
The upper limit of the Mo concentration [Mo] _θ in the cementite grains is preferably 1.20%, more preferably 1.15%, further preferably 1.10%, and further preferably 1.00%.
The Cr concentration [Cr] _θ in the cementite particles is equal to or greater than the Cr content in the chemical composition of the steel sheet. Also, the Mo concentration [Mo] _θ in the cementite particles is equal to or greater than the Mo content in the chemical composition of the steel sheet.

[Method of measuring Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles]
The Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles can be measured by the following method.
A test piece is taken from the center of the steel plate width, and the size of the test piece is 10 mm x 10 mm x plate thickness.

The test piece is subjected to constant current electrolysis using a 10% AA-based solution (a solution containing 10% by volume of acetylacetone and a methanol solution containing 1% by mass of tetramethylammonium chloride).

Specifically, the above-mentioned 10% AA-based solution is prepared. Then, the test piece is subjected to constant-current electrolysis at room temperature with the 10% AA-based solution, with the current density maintained at 20 mA/ ^cm2 . After constant-current electrolysis, the test piece is immersed in an alcohol solution and then subjected to ultrasonic cleaning.

The 10% AA solution used in the constant current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning are suction filtered through a filter with a mesh size of 0.2 μm to extract the residue.

The extracted residue is subjected to chemical elemental analysis. Specifically, the residue is dissolved in acid to obtain a solution. The solution is subjected to chemical elemental analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES) to obtain the Cr mass in the residue and the Mo mass in the residue. Based on the obtained Cr mass in the residue, the Mo mass in the residue, and the total mass of the residue, the Cr concentration [Cr] _θ (mass%) and the Mo concentration [Mo] _θ (mass%) in the residue are obtained. The Cr concentration [Cr] _θ (mass%) in the residue is the value obtained by rounding off the third decimal place of the obtained value to the first decimal place. The Mo concentration [Mo] _θ (mass%) in the residue is the value obtained by rounding off the third decimal place of the obtained value to the first decimal place.

The obtained residue is substantially composed of cementite particles. In other words, the amount of particles other than cementite particles (inclusions and other carbides other than cementite particles) in the residue is negligibly small. Therefore, the Cr concentration and Mo concentration in the residue are regarded as the Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles.

[(Feature 5) Average particle size of cementite particles]
In the steel sheet of this embodiment, the average particle size of the cementite particles is 1.50 μm or less.
If the average particle size of the cementite particles exceeds 1.50 μm, the cementite particles do not dissolve sufficiently during heating in the quenching process in the process of manufacturing a mechanical part using the steel sheet as a material. In this case, the quenchability of the steel sheet is reduced. As a result, the mechanical part manufactured using the steel sheet as a material does not have sufficient strength.

If the average particle size of the cementite particles is 1.50 μm or less, the cementite particles are sufficiently small. In this case, the cementite particles dissolve sufficiently when heated in the above-mentioned quenching process. Therefore, assuming that Features 1 to 4, Features 6 and 7 are satisfied, the hardenability of the steel plate is improved. Furthermore, it also contributes to the spheroidization of the cementite, improving the cold workability of the steel plate.

The upper limit of the average particle size of the cementite particles is preferably 1.45 μm, more preferably 1.40 μm, still more preferably 1.35 μm, and still more preferably 1.30 μm.
In order to improve hardenability, it is preferable that the average particle size of the cementite particles is small. However, if the average particle size of the cementite particles is too small, the hardness of the steel sheet becomes too high due to precipitation strengthening. In this case, the cold workability of the steel sheet decreases. Therefore, the preferable lower limit of the average particle size of the cementite particles is 0.05 μm, more preferably 0.10 μm, more preferably 0.15 μm, more preferably 0.20 μm, more preferably 0.25 μm, and more preferably 0.30 μm.

[Method for measuring average particle size of cementite particles]
The average particle size of the cementite particles can be determined by the following method.
A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate. The surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface measuring 15 mm in the rolling direction and thickness) is defined as the observation surface.

The observation surface is etched using picral solution. Secondary electron images are taken of any five observation fields of the etched observation surface located at a depth of 4/thickness from the surface. Specifically, a scanning electron microscope (SEM) is used to observe the five observation fields at a magnification of 2000x, and the above-mentioned secondary electron images are taken. Each observation field is a rectangle measuring 50 μm in the thickness direction and 60 μm in the rolling direction. The observation field is selected so that the center position of the observation field in the thickness direction is located at a depth of 4/thickness.

In each secondary electron image, cementite particles are identified based on the contrast. Of the identified cementite particles, those that are entirely contained within the observation field of view are taken as the measurement target. In other words, cementite particles that are partly outside the observation field of view are excluded from the measurement target. The area of each cementite particle to be measured is determined, and the circle-equivalent diameter of each cementite particle is determined based on the area. The determined circle-equivalent diameter is taken as the particle diameter of the cementite particle. The particle diameter is determined using well-known image processing software.
The arithmetic mean value of the particle diameters of the cementite particles obtained in the five observation fields is defined as the average particle diameter (μm) of the cementite particles.

[(Feature 6) Maximum particle size of cementite particles]
In the steel sheet of this embodiment, the maximum particle size of the cementite particles is 5.00 μm or less.
If the maximum particle size of the cementite particles exceeds 5.00 μm, during the manufacturing process of a mechanical part made of the steel sheet, the coarse cementite particles in the steel sheet become the starting point of cracks during cold working of the steel sheet. Therefore, sufficient cold workability cannot be obtained. Furthermore, the Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles tend to become concentrated. Therefore, the hardenability of the steel sheet is reduced, and the mechanical part made of the steel sheet does not have sufficient strength.

When the maximum particle size of the cementite particles is 5.00 μm or less, the remaining coarse cementite particles are suppressed after the steel plate is hardened. Therefore, assuming that Features 1 to 5 and Features 7 are satisfied, the hardenability and cold workability of the steel plate are improved in mechanical parts manufactured using the steel plate as a material.

The preferred upper limit of the maximum particle size of the cementite particles is 4.90 μm, more preferably 4.80 μm, even more preferably 4.70 μm, and even more preferably 4.60 μm.

[Method for measuring maximum particle size of cementite particles]
The maximum particle size of the cementite particles can be determined by the above-mentioned [Method for measuring the average particle size of the cementite particles]. Specifically, the maximum particle size of the cementite particles obtained by the [Method for measuring the average particle size of the cementite particles] is defined as the maximum particle size (μm) of the cementite particles.

[(Feature 7) Spheroidization rate]
In the steel plate of this embodiment, when cementite particles having an aspect ratio of 3.0 or less among the multiple cementite particles are defined as spherical cementite particles, the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of the multiple cementite particles, is 75% or more.

If the spheroidization rate is 75% or more, the steel sheet can have remarkably excellent cold workability, provided that Features 1 to 6 are satisfied. Therefore, in the steel sheet of this embodiment, the spheroidization rate is 75% or more.
The spheroidization rate is preferably high. The lower limit of the spheroidization rate is preferably 78%, more preferably 80%, still more preferably 83%, and still more preferably 85%.
A preferred upper limit of the spheroidization rate is 100%. However, if the spheroidization rate is excessively increased, the production cost increases significantly. Therefore, in consideration of industrial production, the upper limit of the spheroidization rate is, for example, 95%, for example, 90%.

[Method for measuring spheroidization rate]
The spheroidization rate can be measured by the following method.
The aspect ratio is determined for each of the multiple cementite particles measured in five observation fields by the above-mentioned [Method for measuring the average particle size of cementite particles]. Specifically, the contour line of the cementite particle is sandwiched between two parallel line segments. At this time, the maximum distance between the two parallel line segments sandwiching the cementite particle is defined as the major axis (μm). Furthermore, the distance between the two line segments when the contour line of the cementite particle is sandwiched between the two line segments parallel to the major axis (i.e., the width in the direction perpendicular to the major axis) is defined as the minor axis (μm).

The aspect ratio (= long axis/short axis) of each cementite particle is calculated based on the obtained long axis (μm) and short axis (μm). Of all the cementite particles in the five observation fields, those with an aspect ratio of 3.0 or less are identified as "spheroidal cementite particles." The ratio of the total number of spherical cementite particles to the total number of the multiple cementite particles being measured is defined as the spheroidal ratio (%). The spheroidal ratio is an integer value obtained by rounding the calculated value to one decimal place.

[Effects of the steel sheet according to this embodiment]
The steel sheet of the present embodiment, which satisfies the above-mentioned features 1 to 7, has sufficient hardenability during hardening in a process for manufacturing a mechanical part using the steel sheet as a material. Furthermore, the steel sheet of the present embodiment has sufficient cold workability.

[Hardenability]
In the steel plate of this embodiment, the fact that sufficient hardenability is obtained means that the steel plate satisfies the following evaluation.

[Heatenability evaluation method]
(A _c1 transformation point measurement)
A test piece having a width of 10 mm, a length of 80 mm, and a thickness of the plate thickness is taken from the center of the plate width of the steel plate of this embodiment. The width direction of the test piece is parallel to the longitudinal direction of the steel plate. If the plate thickness exceeds 3 mm, the thickness of the test piece is adjusted to 3 mm by grinding. The thermal expansion coefficient during heating is measured using a plate Formaster testing machine. The A _c1 transformation point is obtained from the obtained thermal expansion coefficient.

(Basic hardness measurement)
A plate-shaped test piece was taken from the widthwise center of the steel plate, and had a shape of 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness.
The plate test piece is heated at A _c1 transformation point + 100 ° C for 30 minutes using a salt bath. The plate test piece is then immersed in 60 ° C oil in an oil tank and quenched. The quenched plate test piece is cut into two equal parts in the plate width direction. The cut surface is mirror polished. A Vickers hardness test in accordance with JIS Z2244: 2020 is performed at any three points in the center of the plate thickness of the cut surface after polishing. The test force is 98 N. The arithmetic average value of the obtained Vickers hardness is the basic quenched hardness HD0 (HV). The basic quenched hardness HD0 is an integer value obtained by rounding off the first decimal place of the obtained value.

(Hardenability evaluation)
Hardenability is evaluated by the following Hardenability Evaluation Test 1 and Hardenability Evaluation Test 2. When it is determined that sufficient hardenability is obtained in both Hardenability Evaluation Test 1 and Hardenability Evaluation Test 2, the steel plate is determined to have excellent hardenability.

(Hardenability evaluation test 1)
A plate-shaped test piece was taken from the widthwise center of the steel plate, and had a shape of 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness.
The plate-shaped test piece is immersed in a salt bath at A _c1 transformation point + 80 ° C for 10 minutes. The plate-shaped test piece is then removed from the salt bath and immersed in oil at 60 ° C in an oil tank for quenching. The quenched plate-shaped test piece is cut into two equal parts in the plate width direction. The cut surface is mirror-polished. A Vickers hardness test in accordance with JIS Z2244:2020 is performed at any three points in the center of the plate thickness of the cut surface after polishing. At this time, the test force is 98 N. The arithmetic average value of the obtained Vickers hardness is defined as the quenched hardness HD1 (HV). The quenched hardness HD1 is an integer value obtained by rounding off the first decimal place of the obtained value.
When the obtained hardness HD1 is 95% or more of the basic hardness HD0, the steel plate is determined to have sufficient hardenability.

(Hardenability evaluation test 2)
A plate-shaped test piece is taken from the center of the width of the steel plate. The shape of the plate-shaped test piece is 100 mm in the rolling direction of the steel plate, 30 mm in the width direction, and 1.0 mm in the thickness direction. The thickness of the test piece is adjusted by grinding as necessary.

The plate test piece is heated to 1000°C at 400°C/s by applying electrical current, and then immersed in a water tank to perform water quenching. After water quenching, the plate test piece is further immersed in a salt bath at A _c1 transformation point + 80°C for 10 minutes. The plate test piece is then removed from the salt bath and immersed in oil at 60°C in an oil tank to perform quenching. Then, tempering is performed at 200°C for 1 hour.

A bending test piece measuring 50 mm in the rolling direction of the steel plate, 6 mm in the width direction, and 1.0 mm in the thickness direction is prepared from the tempered test piece. A three-point bending test is carried out using the bending test piece, as shown in Figure 1. Referring to Figure 1, the three-point bending test machine comprises two support stands 10 and a pressure wedge 20 placed in the center of the two support stands. The distance D1 between the central axes of the two support stands 10 is 20 mm. The radius of curvature of the tip of the support stand 10 and the radius of curvature of the tip of the pressure wedge 20 are both 3 mm.

In the three-point bending test, the plate test piece 30 is placed on the support table 10. At this time, the plate test piece 30 is placed so that the center position of the plate test piece 30 in the rolling direction coincides with the center position of the distance D1. After the plate test piece 30 is placed, the pressure wedge is moved downward at a moving speed of 5 mm/min to apply a load to the center position of the plate test piece 30. At this time, the movement amount (stroke amount) of the pressure wedge 20 and the load applied to the plate test piece 30 are measured, and the load-stroke curve shown in Figure 2 is created.

In the obtained load-stroke curve, if the hardenability is low, as shown by curve C1 in Figure 2, the load reaches a maximum value and then decreases as the stroke amount increases. On the other hand, if the hardenability is high, as shown by curve C2, unstable fracture occurs before the load reaches a maximum value. When unstable fracture occurs, it is determined that sufficient hardenability has been obtained.

[Cold workability]
In the steel sheet of this embodiment, the fact that sufficient cold workability is obtained means the following evaluation.

[Cold workability evaluation method]
A JIS No. 5 plate-shaped test piece as specified in JIS Z2241:2022 is taken from the center of the width of the steel plate. The thickness of the test piece is 1 mm. The thickness of the test piece is adjusted by grinding as necessary. A V-notch is formed at the center position of the parallel part of the test piece in the longitudinal direction. The opening angle of the V-notch is 45°, and the depth of the V-notch is 2 mm. The depth direction of the V-notch corresponds to the width direction of the parallel part of the test piece. The gauge length is 5 mm including the V-notch part. The longitudinal direction of the test piece is the rolling direction (L direction) of the steel plate.

　A breaking elongation test is carried out at room temperature in air using test pieces. The butt elongation after breaking is measured, and the obtained butt elongation (breaking elongation) (%) is defined as the notch elongation (%). If the obtained notch elongation is 8.0% or more, it is determined that the steel plate has sufficient cold workability.

[Steel plate applications]
The steel plate of this embodiment is suitable as a material for machine parts for automobiles. Examples of machine parts for automobiles include automobile bearings, springs, washers, etc. The steel plate of this embodiment is suitable as a material for machine parts for textile machines. Examples of machine parts for textile machines include knitting needles, etc. The steel plate of this embodiment can be widely used in applications requiring excellent hardenability and excellent cold workability.

[Method of manufacturing steel sheet]
An example of a manufacturing method for the steel plate of this embodiment will be described. The manufacturing method for the steel plate described below is one example for manufacturing the steel plate of this embodiment. Therefore, the steel plate having the above-mentioned configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the manufacturing method for the steel plate of this embodiment.

An example of a method for manufacturing a steel sheet according to the present embodiment includes the following steps.
(Step 1) Material preparation step (Step 2) Hot rolling step (Step 3) Cold rolling step (Step 4) Cold-rolled sheet annealing step In this embodiment, the annealing step is not performed after the hot rolling step and before the cold rolling step.

The main production conditions in the above steps 1 to 4 are as follows:
(Condition 1) Average cooling rate CR1 from the finish rolling temperature to the intermediate temperature MT in process 2: 8.5°C/sec or more (Condition 2) Intermediate temperature MT in process 2: CT+80°C or less and A _c1 transformation point or less (Condition 3) Coiling temperature CT in process 2: 600 to 700°C
(Condition 4) Hot-rolled sheet annealing is not performed. (Condition 5) Cold rolling ratio RR in process 3: 20 to 60%.
(Condition 6) Annealing temperature T1 in step 4: 550 to 750° C.
(Condition 7) Holding time t1 in step 4: 10 to 60 hours

Each process is explained below.

[(Process 1) Material preparation process]
In the material preparation step, a material satisfying characteristic 1 is prepared. The material is produced, for example, by the following method. Molten steel having a chemical composition satisfying characteristic 1 is produced. A slab is produced by a known continuous casting method using the molten steel.

[(Step 2) Hot rolling step]
In the hot rolling process, hot rolling is performed on the prepared material (slab) to produce a steel plate. The hot rolling process includes a rough rolling process in which the material is roughly rolled to produce a rough bar (intermediate steel plate), and a finish rolling process in which the rough bar is finish rolled to produce a steel plate.

In the rough rolling process, the material (slab) is heated in a heating furnace. The heated material is rolled using a rough rolling mill to produce a rough bar. The heating temperature of the material in the rough rolling process is, for example, 1050 to 1300°C. The material is left in the heating furnace for 30 minutes or more, and preferably 60 minutes or more. There is no particular limit to the upper limit of the time spent in the furnace, but it is, for example, 300 minutes.

In the finishing rolling process, the rough bar is further rolled (finish rolling) using a finishing mill to produce steel plate. The finishing mill includes multiple stands arranged in a row. Each stand is equipped with a pair of work rolls. The surface temperature of the steel plate at the outlet side of the stand that lastly rolls down the steel plate among the multiple stands of the finishing mill is defined as the finishing rolling temperature (°C). In this embodiment, the finishing rolling temperature is 800 to 950°C. After finishing rolling, the hot-rolled steel plate is transported to the coiler on the run-out table, and then wound up by the coiler to form a coil. Cooling after finishing rolling is performed in two stages. The average cooling rate CR1 in the first stage cooling, the intermediate temperature MT which is the switching temperature from the first stage cooling to the second stage cooling, and the winding temperature CT will be described later. The coiled steel plate is cooled to room temperature.

[(Step 3) Cold rolling step]
In the cold rolling process, the steel sheet after the hot rolling process is subjected to cold rolling using a cold rolling mill. The cold rolling mill is, for example, a reverse rolling mill consisting of one rolling stand, and the rolling stand includes a pair of work rolls.

In the cold rolling process, cold rolling is performed using the above-mentioned reverse rolling mill to produce cold-rolled steel sheets. The cold reduction rate RR in the cold rolling process will be described later.

In this embodiment, the hot-rolled steel sheet after the hot rolling process is subjected to a cold rolling process without being annealed. In other words, the annealing process is not performed after the hot rolling process and before the cold rolling process. In this manufacturing method, strain is accumulated in the steel sheet in the hot rolling process and the cold rolling process, and the annealing process is performed. As a result, ferrite grains of appropriate size, cementite particles of appropriate size, and appropriate Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite particles are obtained.

[(Step 4) Cold-rolled sheet annealing step]
In the cold-rolled steel sheet annealing process, the cold-rolled steel sheet after the cold rolling process is subjected to an annealing treatment. In the annealing process, the degree of recrystallization of ferrite and precipitation of cementite particles is adjusted by adjusting the annealing temperature T1 and the holding time t1 at the annealing temperature T1.

[Regarding conditions 1 to 7]
In the above steps 1 to 4, the following conditions 1 to 7 are satisfied.
(Condition 1) Average cooling rate CR1 from the finish rolling temperature to the intermediate temperature MT in process 2: 8.5°C/sec or more (Condition 2) Intermediate temperature MT in process 2: CT+80°C or less and A _c1 transformation point or less (Condition 3) Coiling temperature CT in process 2: 600 to 700°C
(Condition 4) Hot-rolled sheet annealing is not performed. (Condition 5) Cold rolling ratio RR in process 3: 20 to 60%.
(Condition 6) Annealing temperature T1 in step 4: 550 to 750° C.
(Condition 7) Holding time t1 in step 4: 10 to 60 hours Each condition will now be described.

[(Condition 1) Average cooling rate CR1]
In the cooling stage of the hot rolling process, the average cooling rate (°C/sec) from the finish rolling temperature to the intermediate temperature MT is defined as the average cooling rate CR1 (°C/sec). The steel sheet of this embodiment is a so-called hypereutectoid steel. Therefore, during cooling from the finish rolling temperature, the steel sheet passes through a temperature range where pro-eutectoid cementite is likely to form (pro-eutectoid cementite formation temperature range).
If the cooling rate in the pro-eutectoid cementite formation temperature range is too slow, coarse pro-eutectoid cementite is formed. The formed coarse pro-eutectoid cementite is likely to remain coarse even after the cold-rolled sheet annealing process is performed. As a result, the maximum particle size of the cementite particles in the steel sheet may exceed 5.00 μm.
In order to suppress the formation of coarse pro-eutectoid cementite, it is necessary to make the cooling rate in the pro-eutectoid cementite formation temperature range sufficiently fast. If the average cooling rate CR1 between the finish rolling temperature and the intermediate temperature MT is 8.5°C/sec or more, the residence time of the steel sheet in the pro-eutectoid cementite formation temperature range is sufficiently short. Therefore, the formation of coarse pro-eutectoid cementite can be suppressed. As a result, the average particle size of the cementite particles becomes 1.50 μm or less, and the maximum particle size of the cementite particles becomes 5.00 μm or less.

The upper limit of the average cooling rate CR1 is not particularly limited. However, due to equipment constraints, the upper limit of the average cooling rate CR1 is, for example, 50.0°C/sec.

[(Condition 2) Regarding intermediate temperature MT]
In the cooling stage of the hot rolling process, the temperature between the finish rolling temperature and the coiling temperature CT, at which the cooling at the average cooling rate CR1 is stopped, is defined as the intermediate temperature MT (°C). If the intermediate temperature MT is higher than CT+80°C or higher than the A _c1 transformation point, the pro-eutectoid cementite formation temperature range may exist in the hot-rolled steel sheet even in the temperature range below the intermediate temperature MT. Therefore, pro-eutectoid cementite is likely to form below the intermediate temperature MT. In this case, the maximum particle size of the cementite particles in the steel sheet may exceed 5.00 μm. If the intermediate temperature MT is below CT+80°C and below the A _c1 transformation point, the pro-eutectoid cementite formation temperature range can be cooled at the average cooling rate CR1. Therefore, the formation of pro-eutectoid cementite can be sufficiently suppressed, and the maximum particle size of the cementite particles in the steel sheet can be set to 5.00 μm or less. In this embodiment, the intermediate temperature MT is higher than the coiling temperature CT. Furthermore, when the intermediate temperature MT satisfies condition 2, the average cooling rate from the intermediate temperature MT to the coiling temperature CT is slower than the average cooling rate CR1.

[(Condition 3) Regarding the winding temperature CT]
In the hot rolling process, the coiling temperature CT affects the spheroidization rate of cementite particles and the Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite particles. If the coiling temperature CT is less than 600° C., the hardness of the hot-rolled steel sheet becomes excessively high, and the subsequent cold rolling process cannot be carried out.

On the other hand, if the coiling temperature CT exceeds 700°C, the Cr concentration [Cr] _θ in the cementite particles exceeds 2.65%, and the Mo concentration [Mo] _θ exceeds 1.30%. Furthermore, the spheroidization of the cementite becomes insufficient, and the spheroidization rate of the cementite particles becomes less than 75%. Furthermore, the maximum particle size of the cementite particles in the steel sheet may exceed 5.00 µm.

If the coiling temperature CT is 600-700°C, steel plates that satisfy characteristics 1 to 7 can be manufactured, provided that other conditions are met.

[(Condition 4) Hot-rolled sheet annealing]
In the manufacturing process of this embodiment, hot-rolled sheet annealing is not performed between the hot rolling process and the cold rolling process. As described above, in this manufacturing method, strain is accumulated in the steel sheet in the hot rolling process and the cold rolling process, and the annealing process is performed. As a result, a ferrite grain size of an appropriate size, cementite particles of an appropriate size, and Cr concentration [Cr] _θ and Mo concentration [Mo] _θ of appropriate concentrations in the cementite particles are obtained. The strain in the hot-rolled steel sheet is smaller than that in the cold-rolled steel sheet. Therefore, when the cementite in the hot-rolled steel sheet is spheroidized by performing hot-rolled sheet annealing, it takes time to spheroidize. In this case, the total annealing heating time of the heating time of the hot-rolled sheet annealing and the heating time in the subsequent cold-rolled sheet annealing process becomes excessively long. Therefore, the ferrite grains and cementite particles become coarse, and the Cr concentration [Cr] _θ and Mo concentration [Mo] _θ of the cementite particles become excessive.

[(Condition 5) Cold rolling rate RR]
In the cold rolling process, the cold rolling reduction ratio RR is defined by the following formula:
Cold rolling rate RR (%) = (1 - (thickness of cold-rolled steel sheet after cold rolling process / thickness of hot-rolled steel sheet before cold rolling process)) x 100

If the cold rolling rate RR is 20% or more, sufficient strain is introduced into the steel sheet. In this case, in the next annealing process, spheroidization of cementite is promoted, and the spheroidization rate becomes 75% or more. On the other hand, if the cold rolling rate RR is less than 20%, the strain introduced into the steel sheet is insufficient. In this case, recrystallization of ferrite does not progress, the steel sheet becomes hard, and the cold workability is deteriorated. In this case, it is difficult to measure the ferrite grain size.
If the cold rolling rate RR exceeds 60%, cold rolling becomes difficult, and therefore the cold rolling rate RR is 60% or less.

If the cold rolling rate RR is between 20% and 60%, steel sheets that satisfy characteristics 1 to 7 can be manufactured, assuming other conditions are met.

[(Condition 6) Annealing temperature T1]
In the cold-rolled sheet annealing process, the annealing temperature T1 adjusts the spheroidization rate of cementite particles in the steel sheet. If the annealing temperature T1 is less than 550° C., the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 75%. Furthermore, the recrystallization of ferrite does not proceed, the steel sheet becomes hard, and the cold workability is deteriorated.

On the other hand, if the annealing temperature T1 exceeds 750° C., the annealing temperature is too high. In this case, the Cr concentration [Cr] _θ in the cementite particles exceeds 2.65%, and the Mo concentration [Mo] _θ exceeds 1.30%. Furthermore, the spheroidization of the cementite becomes insufficient, and the spheroidization rate of the cementite particles becomes less than 75%.

If the annealing temperature T1 is 550-750°C, steel plate satisfying characteristics 1 to 7 can be manufactured, provided that other conditions are met.

[(Condition 7) Holding time t1 at annealing temperature T1]
In the cold-rolled sheet annealing process, the holding time t1 at the annealing temperature T1 affects the spheroidization rate of the cementite particles and the Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite particles. Specifically, if the holding time t1 is less than 10 hours, the cementite is not sufficiently spheroidized.

On the other hand, if the holding time t1 exceeds 60 hours, the holding time is too long. In this case, Cr and Mo are concentrated in the cementite particles, and the Cr concentration [Cr] _θ in the cementite particles exceeds 2.65% and the Mo concentration [Mo] _θ exceeds 1.30%.

If the holding time t1 is between 10 and 60 hours, steel plates that satisfy characteristics 1 to 7 can be manufactured, provided that other conditions are met.

The above manufacturing process allows the production of steel plates that satisfy features 1 to 7.

The effects of the steel plate of this embodiment will be explained in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel plate of this embodiment. Therefore, the steel plate of this embodiment is not limited to this one example of conditions.

Steel plates were manufactured having the chemical compositions shown in Tables 1A and 1B.

Specifically, molten steel was continuously cast to produce a slab. A hot rolling process was carried out on the slab. Specifically, the slab was heated at 1050-1300°C for 240 minutes. The heated slab was rolled in a rough rolling mill to produce a rough bar. The rough bar was then rolled using a finishing rolling mill to produce a steel plate. The finishing rolling temperature for each test number was 800-950°C. The hot-rolled steel plate after finish rolling was wound up into a coil. The coiled steel plate was allowed to cool to room temperature. The average cooling rate CR1, intermediate temperature MT, and coiling temperature CT in the hot rolling process for each test number are shown in the "CR1 (°C/sec)", "Intermediate temperature MT (°C)", and "Coiling temperature CT (°C)" columns in Table 2 (Tables 2A and 2B). In addition, the average cooling rate CR2 from the intermediate temperature MT to the coiling temperature CT was less than the average cooling rate CR1 for all test numbers.

For test numbers 1 to 74, the hot-rolled steel sheets after the hot rolling process were subjected to a cold rolling process without hot-rolled sheet annealing. The cold rolling rate RR in the cold rolling process is shown in the "Cold rolling rate RR (%)" column in Table 2 (Tables 2A and 2B). The cold-rolled steel sheets after cold rolling were subjected to a cold-rolled sheet annealing process. The annealing temperature T1 and holding time t1 are shown in the "T1 (°C)" and "t1 (hours)" columns in Table 2 (Tables 2A and 2B). In the cold-rolled sheet annealing process, the steel sheets were furnace-cooled after the holding time t1. For test number 75, hot-rolled sheet annealing was performed after the hot rolling process and before the cold rolling process. The annealing temperature (°C) and holding time (hours) at the annealing temperature in the hot-rolled sheet annealing are shown in the "Annealing temperature (°C)" and "Annealing time (hours)" columns in Table 2 (Tables 2A and 2B). Steel plates were produced using the above manufacturing process.

[Evaluation test]
The following tests were carried out on the manufactured steel plates having each test number.
(Test 1) Chemical composition measurement test (Test 2) Total area ratio measurement test of ferrite and cementite particles (Test 3) Average ferrite particle size measurement test (Test 4) Average particle size measurement test of cementite particles (Test 5) Maximum particle size measurement test of cementite particles (Test 6) Spheroidization rate measurement test of cementite particles (Test 7) Measurement test of Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles (Test 8) Hardenability evaluation test (Test 9) Cold workability evaluation test Tests 1 to 9 will be described below.

[(Test 1) Chemical composition measurement test]
The chemical composition of each steel plate was measured based on the method described in the above [Method of measuring chemical composition of steel plate]. As a result, the chemical composition of each steel plate was as shown in Tables 1A and 1B.

[(Test 2) Total Area Ratio Measurement Test of Ferrite and Cementite Particles]
Based on the method described in the above [Method for measuring the total area ratio of ferrite and cementite particles in a microstructure], the total area ratio of ferrite and cementite particles for each test number was determined. The obtained total area ratios of ferrite and cementite particles are shown in the "Total area ratio of ferrite and cementite particles (%)" column of Table 3 (Table 3A and Table 3B). In the "Total area ratio of ferrite and cementite particles (%)" column of Table 3 (Table 3A and Table 3B), "≧95" indicates that the total area ratio of ferrite and cementite particles was 95% or more. In any test number, when a remainder other than ferrite and cementite particles was present in the microstructure, the remainder was one or more selected from the group consisting of bainite, martensite, and pearlite.

[(Test 3) Ferrite average grain size measurement test]
The average grain size (μm) of ferrite in the steel sheets of each test number was determined based on the method described in [Method for measuring the average grain size of ferrite] above. The obtained average grain sizes of ferrite are shown in the "Ferrite grain size (μm)" column of Table 3 (Tables 3A and 3B).

[(Test 4) Average particle size measurement test of cementite particles]
The average particle size (μm) of the cementite particles of the steel plate of each test number was determined based on the method described in [Method for measuring the average particle size of cementite particles] above. The average particle sizes of the cementite particles obtained are shown in the "Cementite particle size (μm)" column of Table 3 (Tables 3A and 3B).

[(Test 5) Maximum particle size measurement test for cementite particles]
Based on the method described in the above [Method for measuring maximum particle size of cementite particles], the maximum particle size (μm) of cementite particles of the steel plate of each test number was obtained. If the obtained maximum particle size of cementite particles was 5.00 μm or less, it was indicated as "E" (Excellent) in the "Maximum cementite particle size ≦ 5 μm" column in Table 3 (Tables 3A and 3B). On the other hand, if the maximum particle size of cementite particles exceeded 5.00 μm, it was indicated as "NA" (Not Accepted) in the "Maximum cementite particle size ≦ 5 μm" column in Table 3 (Tables 3A and 3B).

[(Test 6) Measurement test of spheroidization rate of cementite particles]
The spheroidization ratio (%) of the cementite particles of the steel plate of each test number was determined based on the method described in the above-mentioned [Method of measuring spheroidization ratio]. The obtained spheroidization ratios are shown in the "Spheroidization ratio (%)" column of Table 3 (Tables 3A and 3B).

[(Test 7) Measurement test of Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles]
Based on the method described in [Method of measuring Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles] above, the Cr concentration [Cr] _θ (mass%) and Mo concentration [Mo] _θ (mass%) in cementite particles of the steel plate of each test number were determined. The obtained Cr concentration [Cr] _θ and Mo concentration [Mo] _θ are shown in the columns "[Cr] _θ (mass%)" and "[Mo] _θ (mass%)" in Table 3 (Tables 3A and 3B).

[(Test 8) Hardenability Evaluation Test]
In the hardenability evaluation test, (hardenability evaluation test 1) and (hardenability evaluation test 2) were carried out based on the method described in the above-mentioned [Hardenability evaluation method] to evaluate the hardenability of the steel plate of each test number during hardening.

The hardening hardness HD1 obtained in hardenability evaluation test 1 is shown in the "hardening hardness HD1 (HV)" column of Table 3 (Tables 3A and 3B). The "hardening hardness lower limit (HV)" in Table 3 (Tables 3A and 3B) indicates the value of "basic hardening hardness HD0 (HV)" in Table 1B x 0.95. If the hardening hardness HD1 is equal to or greater than the hardening hardness lower limit, it was determined that sufficient hardenability was obtained (shown as "E" in the "hardenability judgment" column in Table 3 (Tables 3A and 3B)). On the other hand, if the hardening hardness HD1 is less than the hardening hardness lower limit, it was determined that sufficient hardenability was not obtained (shown as "NA" in the "hardenability judgment" column in Table 3 (Tables 3A and 3B)).

Furthermore, in hardenability evaluation test 2, if unstable fracture occurred in the load-stroke curve obtained as a result of the three-point bending test, it was determined that sufficient hardenability was obtained (indicated by "E" in the "three-point bending fracture" column in Table 3 (Tables 3A and 3B)). On the other hand, if unstable fracture did not occur in the load-stroke curve obtained, it was determined that sufficient hardenability was not obtained (indicated by "NA" in the "three-point bending fracture" column in Table 3 (Tables 3A and 3B)).

If it was determined that sufficient hardenability was obtained in both hardenability evaluation test 1 and hardenability evaluation test 2, the hardenability of the steel plate with that test number was determined to be excellent.

[(Test 9) Cold workability evaluation test]
The cold workability of the steel sheets of each test number was evaluated based on the method described in the above [Cold workability evaluation method]. The notch elongation obtained is shown in the "Notch elongation (%)" column of Table 3 (Table 3A and Table 3B). If the notch elongation obtained was 8.0% or more, it was determined that sufficient cold workability was obtained. On the other hand, if the notch elongation was less than 8.0%, it was determined that sufficient cold workability was not obtained.

[Evaluation Results]
With reference to Table 1A, Table 1B, Table 2 (Table 2A and Table 2B), and Table 3 (Table 3A and Table 3B), in test numbers 1 to 48, the chemical composition was appropriate and the manufacturing conditions, Conditions 1 to 7, were satisfied. Therefore, the steel sheets with these test numbers satisfied Features 1 to 7. As a result, sufficient hardenability was obtained, and sufficient cold workability was obtained.

On the other hand, in test number 49, the C content was too high. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In test number 50, the Si content was too high. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In test number 51, the Mn content was too low. As a result, sufficient hardenability was not obtained.

In test number 52, the Mn content was too high. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In the test numbers 53 and 54, the Cr content was too high. Therefore, the Cr concentration [Cr] _θ in mass% in the cementite particles exceeded 2.65%, and sufficient hardenability was not obtained.

In test numbers 55 to 57, the Mo content was too high. Therefore, the Mo concentration [Mo] _θ in mass% in the cementite particles exceeded 1.30%, and sufficient hardenability was not obtained.

In test numbers 58 and 59, the chemical composition was appropriate, but the average cooling rate CR1 was too slow. As a result, the maximum particle size of the cementite particles exceeded 5.00 μm. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In test numbers 60 to 62, the intermediate temperature MT was too high. As a result, the maximum particle size of the cementite particles exceeded 5.00 μm. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In the test numbers 63 and 64, although the chemical composition was appropriate, the coiling temperature CT was too high. Therefore, the maximum particle size of the cementite particles exceeded 5.00 μm. In addition, the Cr concentration [Cr] _θ in the cementite particles was too high, and further, the spheroidization rate of the cementite particles was low. As a result, sufficient hardenability and cold workability were not obtained.

In test numbers 65 and 66, although the chemical composition was appropriate, the cold rolling rate RR was too low. As a result, the recrystallization of ferrite did not progress and was not completed. As a result, the ferrite grain size could not be measured (shown as "-" in the "Ferrite grain size (μm)" column in Table 3). As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

Although test numbers 67 and 68 had the appropriate chemical composition, the annealing temperature T1 in the cold-rolled sheet annealing process was too low. As a result, the spheroidization rate of cementite particles was low at less than 75%. Furthermore, the recrystallization of ferrite was not complete, and the ferrite grain size was impossible to measure (shown as "-" in the "Ferrite grain size (μm)" column in Table 3B). As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In the test numbers 69 and 70, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too high. Therefore, the Cr concentration [Cr] _θ in the cementite particles was too high, and further, the spheroidization rate of the cementite particles was low. As a result, sufficient hardenability and sufficient cold workability were not obtained.

In test numbers 71 and 72, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too short. As a result, the spheroidization rate of the cementite particles was too low at less than 75%. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In the test numbers 73 and 74, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too long. Therefore, the Cr concentration [Cr] _θ in the cementite particles was too high. As a result, sufficient hardenability was not obtained.

In the test number 75, the hot-rolled sheet was annealed after the hot rolling process and before the cold rolling process. Therefore, the ferrite grain size and the cementite grain size were too large. Furthermore, the Cr concentration [Cr] _θ in the cementite grains was too high. As a result, sufficient hardenability was not obtained.

The above describes the embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (2)

A steel plate,
The chemical composition is in mass percent:
C: more than 0.90 to 1.30%,
Si: 0.01 to 0.50%,
Mn: 0.20-1.30%,
P: 0.100% or less,
S: 0.100% or less,
Al: 0.100% or less,
Cr: 0.01-0.50%,
N: 0.0150% or less,
Mo: 0-0.400%,
Ni: 0-1.000%,
B: 0 to 0.0100%,
V: 0 to 0.500%,
Nb: 0 to 0.500%, and
Ti: 0 to 0.150%;
The balance is Fe and impurities.
In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more,
The average grain size of the ferrite is 15.0 μm or less,
The Cr concentration [Cr] _θ in mass% in the cementite particles is 2.65% or less, and the Mo concentration [Mo] _θ in mass% in the cementite particles is 1.30% or less,
The average particle size of the cementite particles is 1.50 μm or less,
The maximum particle size of the cementite particles is 5.00 μm or less,
Among the cementite particles, the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, and a spheroidization rate, which is a ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 75% or more.
Steel plate. The steel sheet according to claim 1,
The chemical composition is in mass percent:
Mo: 0.001-0.400%,
Ni: 0.001 to 1.000%,
B: 0.0001 to 0.0100%,
V: 0.001-0.500%,
Nb: 0.001 to 0.500%, and
Ti: 0.001 to 0.150%;
Steel plate.

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